Field of the Invention
The invention is in the fields of mechanical engineering and construction, especially mechanical construction, for example automotive engineering, aircraft construction, shipbuilding, machine construction, toy construction etc.
Description of Related Art
In the automotive, aviation and other industries, there has been a tendency to move away from steel constructions and to use lightweight material such as aluminum or magnesium metal sheets or die-cast parts, or carbon fiber reinforced polymers instead.
The new materials cause new challenges in bonding elements of these materials—especially of bonding flattish objects (such as panels or boards) together or bonding a flattish object and an other object together, such as bonding a flattish object to another object, or bonding a connector to a flattish object.
Difficulties especially arise if objects of different materials are to be connected, such as two materials of the group including steel, aluminum, magnesium, fiber reinforced polymers—together. Conventional rivet connections with metallic rivets firstly suffer from the drawback that the electrochemical potential of some of these materials is strongly different with differences corresponding to several volts, so that there will be substantial galvanic corrosion. Also, connections involving flat objects of fiber reinforced polymers suffer from the additional drawback that the out-of-plane Young's modulus of these materials is very low, and the friction force arising from the compression of the objects between the rivet head and rivet foot does not substantially contribute to the mechanical stability of the connection. (In this text, generally the broadening at the end from which the rivet is accessed for a deformation process is called “head”, whereas the broadening at the other, distal end is called “foot”. In literature, often both ends of the rivet are called ‘heads’)
It has been proposed to use a lacquer on metallic rivets to electrically insulate the metallic rivets from the object they are bonded to. However, lacquer may become brittle over time, especially when subject to long-time mechanical wear due to vibration, or it can dissolve.
For connections between thermoplastic objects, it has further been proposed to shape a rivet shaft as part of one of the objects to be joined and to form a rivet head after positioning relative to the other object by ultrasonic deformation. However, this kind of connection is restricted to bonding thermoplastic materials and not suited for solving the above-mentioned problems.
To solve these problems, the automotive, aviation and other industries have started heavily using adhesive bonds. Adhesive bonds can be light and strong but suffer from the disadvantage that there is no possibility to long-term control the reliability, since a degrading adhesive bond, for example due to an embrittling adhesive, is almost impossible to detect without entirely releasing the bond.